Electrostatography, which broadly includes the forming and developing of electrostatic image patterns either with or without light, has become a major field of technology. It perhaps is best known through the use of electrophotographic office copying machines. With the increased use of plain paper copiers, dry developers have enjoyed an increased popularity over liquid toners. Along with the increased use of dry developers magnetic brush development has become increasingly popular as opposed to cascade development.
Magnetic brush development uses ferromagnetic carrier particles, usually coated with a resin which aids in triboelectrically charging the toner. A magnet carries the developer mixture of toner and carrier particles and the magnetic field causes the carrier particles to align like the bristles of a brush. As the developer brush contacts the electrostatic latent image formed on a photoconductive or dielectric surface, toner particles are drawn away from the carrier particles by the oppositely charged electrostatic image. This alignment of the carrier particles toward the photoconductor surface will decrease the distance between carrier particles and the photoconductor surface. This provides the-effect of a development electrode with very close spacing to the photoconductor surface, which results in high quality tonal characteristics in the reproduced image. The higher the conductivity of the ferromagnetic carrier particles, the greater will be their effectiveness as a development electrode. The copying process may be completed by transferring the toned image to paper where it is fused and fixed, for instance, by pressing the paper with a heated roller.
Also, it is well-known that for high speed copiers a rapid development rate is necessary if high density images are to be achieved. Such rapid development rates are facilitated by using carrier particles having a highly conductive carrier core. Ferromagnetic carrier particles such as, for example, stainless steel are desirable for this purpose due to their excellent conductivity and stability.
Unfortunately, stainless steel typically has a layer of scale, or oxide, on the surface due to its method of manufacture which serves to decrease its conductivity slightly. It is therefore desirable to remove this scale to make the stainless steel more conductive and cause a corresponding increase in development rate.
It is known that certain acids may be used to remove oxides from metals. For example, U.S. Pat. No. 4,310,611 to Miskinis discloses passivating stainless steel electrographic carrier particles by treatment with nitric acid. Preferably, the stainless steel particles are treated first with a 1% HF solution for about 12 minutes, rinsed in a distilled water wash, and then treated with a 20% HNO.sub.3 solution at 65.degree. C. for about 20 minutes followed by a distilled water wash and a methanol rinse. The chemicals for this process are relatively expensive, and also relatively hazardous to work with, particularly at elevated temperatures.
Also, a variety of acid treatments have been disclosed for treating steel to increase corrosion resistance. A great deal of this work has utilized various acids, including oxalic acid, to apply protective coatings on to various metal surfaces.
For example, U.S. Pat. No. 4,316,752 to Kronstein discloses a method for improving the corrosion resistance of carbon steel or galvanized steel by treating the metal surface with a dilute aqueous oxalic acid solution having a temperature of 65.degree. C. to 90.degree. C., to form a passivated layer.
Japanese published Appln. No. J59-162,224 discloses a method for increasing the strength of stainless steel rods involving submersing the rods in liquid oxalic acid. The rods are then press-formed into bolts and given a solid solution heat treatment, allegedly resulting in bolts having higher strength than those made using ordinary methods.
A great deal of work has involved depositing oxalate coatings onto metal surfaces for, inter alia, corrosion resistance. For example, see U.S. Pats. Nos. 1,315,017; 1,696,036; 1,895,568; 1,895,569; 1,911,537; 2,550,660; 3,121,033; 3,806,375; and 3,879,237.
U.S. Pat. Nos. 3,632,512 and 3,718,594 to Miller discloses treating ferromagnetic carrier particles in an aqueous acid solution followed by rinsing and controlled drying to form a thin uniform layer of iron oxide on the iron particles which apparently improves particle conductivity. Miller states that acids such as sulphuric, other mineral acids, and certain organic acids, may be used to form this layer.
U.S. Pat. No. 4,247,597 to Russell, Jr. discloses treating ferromagnetic carrier particles with a carboxylic acid solution and allowing the solvent to evaporate, leaving a protective monomolecular layer of a non-halogenated carboxylic acid. Russell teaches that an anhydrous solution of carboxylic acid should be utilized to apply this acid layer, using a solvent such as methanol or methyl ethyl ketone.
U.S. Pat. No. 5,096,797 to Yoerger discloses treating strontium or barrium ferrite carrier particles with an aqueous acid solution to dissolve any loose strontium or barium oxide dust. One of the acids listed in this patent is oxalic acid. There is no suggestion, however, of using an aqueous oxalic acid solution to remove scale and improve the conductivity of stainless steel carrier particles.
It would be desirable to find a simple method to remove scale from stainless steel, and thus increase the conductivity of the stainless steel, using fewer and/or cheaper chemicals, and preferably at room temperature.